Question

5.
How the following phenomena help in the ascent of sap in the plants ?
(i) Diffusion :
(ii) Osmosis:
(iii) Capillary action :
(iv) Root pressure :
(v) Transpiration pull:​

Answer 1

Answer:

The mechanism or process of ascent of sap occurs due to activity of living cells. Atmospheric Pressure is what responsible for this. Ascent of sap is the upward movement of water absorbed by roots and is distributed to all parts of plant in order to reach to. topmost parts of plant, water move upward through stem.

Explanation:

1. Diffusion: It is the process of movement of molecules from a region of higher concentration (say CO2 from outside causes gas to diffuse to move into leaf) to a region of lower concentration. Similarly, in ascent of sap due to concentration gradient diffusion helps in movement of minerals and ions into roots.
2. Osmosis: It is the transport of solvent through semi-permable membrane that seperates two solutions of differing solute concentrations. In ascent of sap, the cells in root hair are deficient in water because they continuosly supply tha absorbed water to stems and leaves. Hence, the concentration of water in root hair less than water present in soil creating a concentration gradient allows root hair cells to uptake water actively by process of. osmosis.
3. Capillary Action: When water rises in a capillary tune because of physical forces is known as capillary force or capillary action. In ascent of sap, the same way in stem the water rises upto a certain height due to this action only.
4. Root Pressure: Various ions from soil are actively transported into vascular tissues, water follows the potential gradient and increases the pressure inside xylem This positive pressure is known as Root Pressure. In ascent of sap, this activity pushes the water upwards all along the stem. However, this pressure is not that effective due to some reasons like it's magnitude is very low.
5. Transpiration pull: The pulling force that generates on xylem water column is known as Transpiration pull. It is also known as suction force. In ascent of sap, it's magnitude is enough to lift water column to any heights. Transpiration generates pressure in the xylem of leaves to pull the water upward. Dixon and Jolly’s cohesion- tension theory or transpiration pull is the most accepted theory for upward movement of water in the xylem. Some physical properties of water such as cohesion (the tendency of water molecules to cling to one another), adhesion (attraction of water molecules with the walls of xylem)and surface tension support the formation of the water column in the xylem vessel and their upward movement.

Conclusion: All these above terms mentioned are responsible for ascent of sap.